Programming · by Kabir Khan · 2026-10-10
Java 25 virtual threads are quietly changing how we write concurrent backend code
Been migrating an old thread-pool-per-request service to virtual threads (Project Loom, stabilized a while back and still improving in Java 25) and the mental model shift is bigger than expected. Blocking code that used to need careful pooling just... works now, and scales to tens of thousands of concurrent connections without the usual thread-starvation headaches. Anyone hit weird edge cases with ThreadLocal, synchronized blocks pinning carrier threads, or monitoring tools that assume platform threads?
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Gentle Sparrow · 0 likes
the synchronized pinning thing bit us hard, had a legacy library using synchronized blocks everywhere and it tanked our throughput untill we found it with async-profiler
Tiago Costa · 0 likes
yeah pinning is the #1 gotcha everyone finds the hard way, JEP 491 in newer releases helps a lot with this specific case though
Léa Bernard · 0 likes
our APM vendor straight up didnt support virtual threads properly for like a year after launch, thread dumps were useless garbage
Amelia Hughes · 0 likes
this is the biggest quality of life improvement to java backend dev since... honestly maybe ever? no more reactive spaghetti just to get concurrency
Faisal Al Saud · 0 likes
switched a spring boot monolith over, gc pressure actualy went down bc we werent allocating massive thread pools anymore. wasnt expecting that
Mateo Cruz · 0 likes
ThreadLocal usage needs auditing hard before you migrate, we had some nasty memory growth from thread locals that assumed thread reuse semantics
Curious Lantern · 0 likes
still think people underestimate how much simpler this makes onboarding juniors, dont have to teach the whole reactive/future composition mental model anymore
Jasmine Cruz · 0 likes
big agree, explaining CompletableFuture chains to new grads was always rough
Urban Panda · 0 likes
we tried it for a cpu bound workload by mistake lol, obviously no benefit there since its all about IO blocking, lesson learned